1. Field of the Invention
The present invention relates to mounting or support systems and, more particularly, to mounting or support systems for supporting elongate structural members found in offshore applications and subject to dynamic axial and lateral loads.
2. Description of the Prior Art
As the search for oil and gas is extended to progressively deeper offshore waters, greater demands are placed upon the structural components used in those activities. For example, marine riser systems may extend thousands of feet from a wellhead assembly to an offshore platform or vessel. These riser systems, which are sections of tubular components joined together, e.g., by threads, can be subjected to very high dynamic forces that act both axially and laterally on the riser strings. While typically the riser strings are constructed of steel, in an attempt to reduce hang-off loads and/or diminish static and cyclic bending stresses, as well as transmitted moments that generally increase with riser size and/or wave action, titanium alloy components are being incorporated into the riser strings. While steel and other ferrous-based riser strings and offshore components are effectively protected from the effects of corrosive seawater environments by sustained application of cathodic protection, if titanium alloy components are incorporated into the riser strings, it may be necessary to totally isolate impressed cathodic currents/potentials from these components to avoid long-term hydrogen adsorption and damage. Additionally, it is also prudent to preclude the detrimental effects of galvanic coupling between mating or adjoining dissimilar riser system alloys exposed to seawater and/or produced fluid brine through electrical isolation.
For dynamic riser systems, flex joints comprised of interspersed steel and rubber-layer laminate flex elements provide electrical isolation at the riser topside or subsea termination point. Alternative means of electrical isolation in dynamic riser systems using traditional, common electrical insulation materials, such as ceramics and polymers, has been largely unsuccessful because of the undesirable properties offered by these materials. Although ceramics possess elevated compressive strength, they are highly susceptible to physical damage and cracking due to their intrinsic brittleness and low toughness. Additionally, their high stiffness (high modulus) and low shear strength require extremely tight dimensional tolerances and close fit, which are often not achievable in the interface cross-section sizes required in riser systems. Although thermoset and thermoplastic polymers exhibit reasonable ductility, toughness, and durability, and are readily applied as coatings or sheet forms, their compressive bearing, creep, and fatigue strength properties are generally too low for these dynamic systems and rapidly diminish with increasing service temperature.
Over and above the problems of electrically isolating the titanium components from the steel components, dynamic loading on the titanium components can become a severe problem. Typically, elongate titanium alloy structural components, such as risers, are supported by steel support pods or sockets. Because of the cyclic lateral forces exerted on the elongate titanium components, there is a tendency for the titanium components to undergo fretting and fatigue failure generally at the junction of the titanium structural component and the steel support pod or socket. Thus, aside from the problems associated with electrical isolation between the steel components and the titanium components, there is also the problem of diminished fatigue life in the titanium component as a result of fretting at the juncture of the titanium component and the steel support.
It is therefore an object of the present invention to provide a mounting system for structural members used in offshore applications and subjected to dynamic loading.
Another object of the present invention is to provide a mounting system for elongate structural members used in offshore applications that permits electrical isolation of the structural member from associated structures such as offshore platforms.
Another object of the present invention is to provide a mounting system for supporting elongate structural members used in offshore applications that minimizes fretting of the elongate structural member due to dynamic lateral loading.
The above and other objects of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims.
In accordance with the present invention, there is provided a mounting system for elongate, metallic structural members used in offshore applications, e.g., titanium tubing, that are subject to dynamic forces. The mounting system includes a support assembly, the support assembly including a socket that at least partially encircles the structural member. There is an attachment for securing the support assembly to a positioned structure such as an offshore platform. A sleeve is received in the socket, the sleeve being disposed in surrounding relationship to the structural member. The sleeve comprises an electrically nonconductive, fiber-reinforced polymer composite having a through-thickness compressive strength above 25 ksi at temperatures at least as high as 120xc2x0 C. and an S-N fatigue life under cyclic compressive through-thickness loading in excess of 1 million cycles at maximum compressive stress levels of xe2x89xa620 ksi.